1. Field of the Invention
This invention relates to a range detecting method and apparatus, and more particularly, it is concerned with the range detecting method and apparatus of a type such that a distance to an object is measured by electrically detecting a relative quantity of discrepancy in space between two images in utilization of the distance measurement principle in a base line double-image coincidence type distance measurement meter. More specifically, the invention relates to a range detecting method and apparatus of a type, in which image scanning signals concerning the two images are obtained by scanning the abovementioned two images, based on which signals a relative quantity of spatial discrepancy between the two images may be detected.
2. Description of the Prior Art
There have so far been proposed various types of electric double-image coincidence detection type range detecting method and apparatus as outlined in the preceding, or automatic focus adjusting devices for photographic cameras, etc. using such range finding devices. Describing more specifically, this range finding device is of such a type that two images of an object to be formed by a range finding optical system with a relative positional parallax corresponding to the object distance are received by photo-electric light receiving means, and quantities of relative positional parallax in these two images are found from outputs of the light receiving means obtained at this time, thereby calculating a distance to the object for the range finding.
For example, according to Japanese Patent Publication No. 48-5733 (published Feb. 20, 1973, for the invention of "An Automatic Focus Adjusting Device in a Camera" in the name of Canon Kabushiki Kaisha), there is proposed a range detecting method and apparatus in the form of an automatic focus adjusting device, wherein a pair of photo-conductive elements which are so constructed that their resistance values may vary in accordance with positional changes in an image on the light receiving surfaces thereof are juxtaposed, and then images of the one and the same object are formed on these elements by means of a range finding optical system comprising a pair of focussing lenses fixedly disposed at a certain length on the base line so that a distance to the object may be detected by finding a difference in the resistance values between these two elements by utilizing the principle that a relative quantity of discrepancy in the image forming position on each element corresponds to a distance to the object.
In this disclosed device, however, the photo-conductive element per se is of a very peculiar structure, on account of which there inevitably take place various inconveniences in using a pair of these elements such that coincidence of the response characteristics in both elements in an ideal state becomes highly difficult, as the result of which precision in the detection becomes deteriorated due to increase in error signal, and false determination in distance will arise in the range detection due to coincidence of the resistance values in both elements in spite of the relative positions of the images on both elements being actually discrepant.
With a view to solving such a problem, there has been proposed a range detecting device, or an automatic focus adjusting device, in which image scanning signals on the two images are obtained by scanning them in utilization of a photo-electric light receiving means, and then by finding a quantity of relative positional parallax in these two images from the two image scanning signals, the object distance is calculated, or the focus adjustment of the objective lens system in a camera with respect to the object is automatically attained by utilizing informations concerning a relative positional parallax in these two images.
For example, U.S. Pat. No. 3,898,676, (filed Dec. 20, 1975, granted to Hosoe et al. for an invention entitled "Distance Detecting Device" assigned to Canon Kabushiki Kaisha) teaches an automatic focus adjusting device of a construction, wherein arrays of photo-sensors are used as the photo-electric light receiving means for receiving the two images, and, by driving these photo-sensor arrays simultaneously, a photo-electric output of each photo-sensor in the arrays is obtained in a timed sequence to thereby scan the two images simultaneously in a purely electrical manner, and image scanning signals to be obtained on these two images at this time are converted to wave form signals through low pass filters, after which these wave form signals are introduced into a phase discriminator to detect a phase difference between these image scanning signals, and a servomotor is actuated by an output from the phase discriminator to cause are objective lens system to shift along its optical axis, in association with which one of the two images is shifted with respect to the other, whereby "in-focus position" of the objective lens system to the object is determined with a point where the phase difference between the image scanning signals for the two images becomes zero, in other words, a point where the relative positional parallax of the two images becomes zero.
Also, according to Laid-Open Japanese Patent Application No. 51-45556 (Laid-Open on Apr. 19, 1976 for the invention of "Method and Apparatus for Distance Detection", there is proposed a method and an apparatus for detecting distance of an object, which is constructed in such a manner that self-scanning image sensors (a kind of photo-sensor array) are utilized as the photo-electric light receiving means for receiving two images, that the two images are repeatedly scanned by these image sensors, at which time coincidence and non-coincidence of the image scanning signals on the two images to be obtained from the image sensors are detected by means of a coincidence detection circuit, while a timing for commencing scanning of one of the image sensors is varied by a variable delay circuit against a timing for commencing the scanning of the other image sensor, and that the relative positional parallax quantities of the two images, i.e., the object distance is made known directly from a lagged quantity between the timings for starting the scanning operations by the two image sensors which has been found upon detection of the coincidence of the abovementioned image scanning signals by the abovementioned coincidence detection circuit.
In the methods and devices as disclosed in these U.S. Pat. No. 3,899,676 and Laid-Open Japanese Patent Application No. 51-45556, the two images of an object to be formed by the range finding optical system are scanned purely electrically in utilization of photo-sensor arrays or image sensors known as, in particular, Photo-Diode Array, CCD (Charge Coupled Device), or BBD (Bucket Brigade Device), and so on, and the image scanning signals concerning the two images to be obtained at this time are used for distance detection or focus detection. In particular, since the image is scanned in a purely electrical manner utilizing the photo-sensor arrays or image sensors, accurate signal corresponding exactly to the image pattern can be used, on account of which further improvement in precision of the distance detection or the focus detection can be expected.
However, these methods and apparatuses as have heretofore been proposed contain therein many problems still to be solved such as, for example, a concrete method for processing the abovementioned image scanning signals as one aspect, hence their reduction to practice is far-reaching.
For example, in the device proposed in the above-described U.S. Pat. No. 3,899,676, as already mentioned above, the image scanning signals are converted to the waveform signals by causing them to pass through low pass filters, after which the waveform signals are introduced into the phase discriminator, where detection is conducted to find whether any phase difference has occurred between the two signals. In this case, since the phase discriminator deals with the waveform signals, in particular, the overall construction of the device becomes complicated, and, since its reliability is lacking, it is not possible to accurately detect the phase difference when it is extremely small, on account of which the operation of the phase discriminator as the automatic focussing device is unavoidably inaccurate.
Also, in the device as proposed in the above-discussed Laid-Open Japanese Patent Application No. 51-45556, there is merely adopted a differential amplifier or a combination of the differential amplifier and a comparator as the circuit for detecting coincidence and non-coincidence of the two image scanning signals. Considering, however, that the signals which the circuit deals with are time-sequential signals to be produced, as an output, from the image sensors, it is almost impossible to carry out detection of the coincidence and non-coincidence of the image scanning signals with such simple circuit construction.
Furthermore, in the method and apparatus as proposed here, the scanning start timing of one of the image sensors is caused to vary with respect to the scanning start timing of the other image sensor by the use of a variable delay circuit, and a discrepant quantity between the scanning start timings for these two image sensors is taken as the object distance. However, as has so far been well recognized, in utilizing the self-scanning type image sensors such as the photo-diode array of the charge accumulation type, CCD, or BBD, etc., if the scanning start timing, i.e., the timing for imparting the start pulse, is varied, the integration time, i.e., the effective light receiving time, also varies with the result that the level of the output signal varies. Accordingly, in the proposed method and device, since the scanning start timing of one of the image sensors is caused to vary by the variable delay circuit with respect to the scanning start timing of the other image sensor, the two image scanning signals to be obtained there are resulted from extreme level variations, so that, even if it is attempted to compare these two image scanning signals for detection of coincidence, such is apparently impossible. After all, it is not at all certain to attain accurate distance detection.